Disorders of bile duct development constitute a major cause of morbidity and mortality in the pediatric population, yet little is understood regarding the genetic signaling and molecular control of bile duct formation during development. The gene Jagged1, encoding a ligand in the Notch intercellular signaling pathway, has recently been found to be involved in this process. Jagged1 is expressed in the ductal plate, or primitive bile duct cells, during development as studied by in situ hybridization and immunohistochemistry with antibodies to proteins found specifically in the ductal plate. In addition, patients with a mutation or deletion of one copy of Jagged1 have Alagille syndrome, a disorder characterized by cholestasis and paucity of the intrapepatic bile ducts, along with abnormalities of other organ systems. Based on these data, we hypothesize that jagged1 and the Notch pathway have an integral role in signaling during bile duct development and we propose to use a dual approach to define further their functions in this complex process. The first specific aim of the proposal is to use the approach of conditional gene targeting to create a liver-specific disruption of Jagged1 in a mouse model. In this way, it will be possible to examine the effect of loss of Jagged1 during liver and bile duct development without disturbing its activity in any other organ systems. We hyposthesize that the liver-specific jagged1 knockout mouse will demonstrate a severe abnormality of liver and bile duct development. Specific aim two entails an in depth study of the knockout mouse phenotype, including detailed histologic studies and analysis of ductal plate development through the use of immunohistochemical stains. The third aim of the proposal involves the use of a unique bipotential mouse hepatoblast cell line to study signaling in the Nothch pathway during liver and bile duct differentiation. A detailed analysis of Notch gene expression in this cell line provide the basis for transfection experiments that will perturb the signaling pathway. This proposal provides an opportunity to define the role of crucial signaling molecules in bile duct development. The combined approach of a conditional knockout animal model in parallel with a cell culture system will not only elucidate the function of the Notch pathway in liver and bile duct development, but will also identify candidate genes for other bile duct disorders.